Recently, technology that utilizes a spin transfer torque switching in which a current flowing in a magnetic material causes a magnetization reversal has been drawing attention as a high density nonvolatile memory device. A high density MRAM (Magnetoresistive Random Access Memory) is realized by using such a spin transfer torque switching. The memory cell of the MRAM includes a magnetic tunnel junction (MTJ) device. The MTJ device has a stacked structure in which a storage layer and a reference layer are stacked with a thin tunneling barrier layer interposed. To realize the high density MRAM, it is necessary for the MTJ device to have a size of several tens of nm.
However, materials that are used in the MTJ device such as magnetic metals like cobalt (Co), iron (Fe), etc., generally are materials on which dry etching is not easily performed. Accordingly, the angle of the sidewall of the MTJ device cannot have a shape that is substantially perpendicular when using conventional dry etching (e.g., RIE (Reactive Ion Etching)). For example, in conventional dry etching, the MTJ device has a shape in which the width is wider downward. Generally, in a MTJ device with perpendicular anisotropy, the storage layer is arranged lower than the reference layer to improve the magnetic properties. In the case where the width of the storage layer is larger than the width of the reference layer, the switching properties due to the spin transfer torque undesirably degrade because the leakage magnetic field from the reference layer acts nonuniformly on the storage layer.
Because the MTJ device has a structure in which the storage layer and the reference layer are stacked with the thin tunneling barrier layer interposed, the distance between the storage layer and the reference layer is short. Therefore, in the case where the MTJ device is patterned using dry etching, there is a possibility that redeposition of metal may straddle the tunneling barrier layer to adhere to the storage layer and the reference layer at the sidewall of the MTJ device. In such a case, the storage layer and the reference layer may be shorted and the MTJ device may undesirably become defective because a leak current path forms between the storage layer and the reference layer due to the redeposition. Therefore, the yield of the MTJ device unfortunately decreases.